simple_232. 用栈实现队列.java

//https://leetcode-cn.com/problems/implement-queue-using-stacks/submissions/
//pop & peek；若s2不空，只操作s2即可
class MyQueue {
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();

    public MyQueue() {

    }
    
    public void push(int x) {
        s1.push(x);
    }
    
    public int pop() {
        //保证s2非空
        peek();
        return s2.pop();
    }
    
    public int peek() {
        if(s2.isEmpty()) {
            while(!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        }
        return s2.peek();
    }
    
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */